Polyimide Precursor, Polyimide Precursor Composition, Polyimide Film, Method for Preparing the Same, and Use Thereof

ABSTRACT

Provided are a polyimide precursor, a polyimide precursor solution, a polyimide film, a method for preparing the same, and a use thereof. According to an exemplary embodiment of the present invention, a polyimide film having excellent thermal resistance and satisfying transparency and a low coefficient of linear thermal expansion may be provided, and thus, may be usefully applied in a flexible display field requiring high dimensional stability and the like.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No. 10-2020-0085890 filed Jul. 13, 2020, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The following disclosure relates to a polyimide precursor, a polyimide precursor solution, a polyimide film, a method for preparing the same, and a use thereof.

Description of Related Art

Polyimide is conceived as a high thermal resistant, light, and flexible material. As a resin having excellent thermal dimensional stability in the polyimide field, aromatic polyimide is attracting attention. A polyimide film which is a molded body composed of aromatic polyimide having a rigid and linear chemical structure is widely used in the field requiring high thermal dimensional stability (low coefficient of linear thermal expansion) such as a base film of a flexible printed wiring board and an interlayer insulating film of a semiconductor. However, since aromatic polyimide having a low coefficient of linear thermal expansion is strongly colored by intramolecular conjugation and intramolecular/intermolecular charge transfer interaction, it is difficult to apply the aromatic polyimide in optical applications. In addition, since polyimide has a very strong intermolecular force, it lacks processability.

Meanwhile, a flexible device is manufactured by a method of applying a polyimide precursor composition on a conveyance board and curing the composition to form a film, completing a device by a subsequent process such as thin film transistor (TFT) and organic film deposition, and then detaching the completed device from the conveyance board. The flexible device involving a high temperature process as such requires high thermal resistance at a high temperature. In particular, when a thin film transistor process using a low temperature polysilicon (LTPS) is used, a process temperature may be close to 500° C., and thus, the polyimide film which is formed as a film on a conveyance board should not undergo thermal decomposition by hydrolysis even during a process at high temperature and satisfy high thermal resistance. In addition, transparency after processing as well as storage stability should be secured.

Thus, development of a new polyimide which may satisfy high thermal resistance and also prevents hydrolysis to represent excellent chemical resistance and storage stability and improve optical/mechanical characteristics is needed for manufacturing a flexible device.

SUMMARY OF THE INVENTION

An embodiment of the present disclosures provides a polyimide precursor and a polyimide precursor solution for providing a polyimide film having excellent thermal resistance and satisfying transparency and a low coefficient of linear thermal expansion and a method for preparing the same.

Another embodiment of the present disclosures provides a method for preparing a polyimide film using the polyimide precursor solution.

Another embodiment of the present disclosures provides a polyimide film which may be usefully applied to the field requiring high dimensional stability and a multilayer structure including the same.

Still another embodiment of the present disclosures provides a photoelectric device and a flexible display including the polyimide film.

In one general aspect, a polyimide precursor solution includes: a polyimide precursor derived from a tetracarboxylic acid dianhydride represented by Chemical Formula 1:

wherein

Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl;

R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring; and

n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.

In the polyimide precursor solution, the tetracarboxylic acid dianhydride may be selected from compounds represented by the following Chemical Formulae 2 to 5:

wherein

R¹, R², R′, n, and m are as defined in the above Chemical Formula 1.

In the polyimide precursor solution, the polyimide precursor solution may include a polymerization component including the tetracarboxylic acid dianhydride represented by Chemical Formula 1 and a diamine; and an organic solvent.

In the polyimide precursor solution, the diamine may include a unit represented by the following Chemical Formula 6:

wherein

R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl; and

p is an integer of 1 or 2.

In the polyimide precursor solution, the organic solvent may be selected from amides.

In the polyimide precursor solution, the organic solvent may be N,N-diethylacetamide, N,N-diethylformamide, N-ethylpyrrolidone, N,N-dimethylpropionamide, N,N-diethylpropionamide, or a combination thereof.

In the polyimide precursor solution, the tetracarboxylic acid dianhydride selected from compounds represented by the following Chemical Formulae 7 and 8 may be further included as the polymerization component:

wherein

Q³ and Q³ are a single bond, —O—, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, phenylene, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl.

In another general aspect, a polyimide precursor derived from a tetracarboxylic acid dianhydride represented by Chemical Formula 1 and a diamine is provided.

The polyimide precursor may include a repeating unit represented by the following Chemical Formula a:

wherein

Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl;

R^(a) and R^(b) are independently of each other hydrogen or C1-C10 alkyl;

R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring;

R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl;

p is an integer of 1 or 2; and

n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.

The polyimide precursor may include 10 to 100 mol % of the repeating unit represented by Chemical Formula a, based on total repeating units.

In another general aspect, a polyimide film may include a repeating unit represented by the following Chemical Formula b:

wherein

Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl;

R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring;

R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl;

p is an integer of 1 or 2; and

n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.

The polyimide film may further include a repeating unit represented by the following Chemical Formula c or d:

wherein

R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl; and

p is an integer of 1 or 2.

The polyimide film may have a coefficient of thermal expansion (CTE) of 50 ppm/° C. or less at 100 to 450° C.

The polyimide film may have YI in accordance with ASTM E313 of 15 or less, a haze in accordance with ASTM D1003 of 2 or less, and a total light transmittance in a region of 380 to 780 nm in accordance with ASTM D1746 of 80% or more.

The polyimide film may have a modulus in accordance with ASTM D882 of 5.0 or more and an elongation of 15% or more.

The polyimide film may have a coefficient of thermal expansion (CTE) of 50 ppm/° C. or less at 100 to 450° C.; YI in accordance with ASTM E313 of 15 or less; a haze in accordance with ASTM D1003 of 2 or less; an average light transmittance in a region of 380 to 780 nm in accordance with ASTM D1746 of 80% or more; and a modulus in accordance with ASTM D882 of 9.0 or less and an elongation of 15% or more.

In another general aspect, a multilayer structure includes the polyimide film described above.

In another general aspect, a photoelectric device includes the polyimide film described above as a flexible substrate.

In another general aspect, a flexible display includes the polyimide film described above as a flexible substrate.

In another general aspect, a method for preparing a polyimide film includes: applying and coating the polyimide precursor solution described above on a substrate and then performing a heat treatment.

In the method for preparing a polyimide film, the heat treatment may include a first heat treatment performed at 100° C. or lower; a second heat treatment performed at higher than 100° C. and 300° C. or lower; and a third heat treatment performed at higher than 300° C. and 500° C. or lower.

In the method for preparing a polyimide film, the polyimide precursor solution may include a solid content of 10 to 13 wt %, based on a total weight.

Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.

DESCRIPTION OF THE INVENTION

In this specification, unless otherwise defined, all technical terms and scientific terms have the same meanings as those commonly understood by a person skilled in the art to which the present invention pertains. The terms used herein are only for effectively describing a certain specific example, and are not intended to limit the present invention.

The singular form used in the present specification may be intended to also include a plural form, unless otherwise indicated in the context.

In addition, units used in the present specification without particular mention is based on weights, and as an example, a unit of % or ratio refers to a wt % or a weight ratio, and the wt % refers to a wt % of any one component in a total composition occupied in the composition, unless otherwise defined.

In addition, the numerical range used in the present specification includes all values within the range including the lower limit and the upper limit, increments logically derived in a form and span in a defined range, all double limited values, and all possible combinations of the upper limit and the lower limit in the numerical range defined in different forms. Unless otherwise defined in the specification of the present invention, values which may be outside a numerical range due to experimental error or rounding of a value are also included in the defined numerical range.

The term “comprising” in the present specification may be an open-ended term implying further inclusion of other components, not exclusion of other components, unless otherwise stated.

The term “derived” in the present specification refers to at least any one of functional groups of a compound being modified, and specifically may include a modified form or leased form of a functional group and/or a leaving group of a compound according to a reaction. In addition, when structures derived from compounds different from each other are the same, it may include a case in which a structure derived from any one compound has the same structure as that is derived from any other compound.

The term “polyimide precursor solution” in the present specification refers to a composition for preparing polyimide, and specifically, a polyimide precursor refers to a polymer including a structure unit having an amic acid moiety and may be equivalent to a polyamic acid. In addition, the polyimide precursor solution may be also used as a composition for preparing polyamideimide.

The term “polyimide film” in the present specification is a molded body of polyimide derived from a polyamide precursor solution and may be equivalent to polyimide.

The term “halogen” in the present specification refers to a fluorine (F), chlorine (Cl), bromine (Br), or iodine (I) atom.

The term “alkyl” in the present specification is an organic radical derived from an aliphatic hydrocarbon by removal of one hydrogen and may include both linear and branched forms.

The term “alkoxy” in the present specification is represented as *—O-alkyl, and the alkyl is as defined above.

The terms “haloalkyl” and “haloalkoxy” in the present specification refer to the alkyl or alkoxy having one hydrogen replaced by a halogen.

The term “aryl” in the present specification is an organic radical derived from an aromatic hydrocarbon by removal of one hydrogen, including a monocyclic or fused cyclic system, and even a form of plural aryls connected by a single bond.

The present inventors intensively studied polyimide which is optically transparent and has excellent thermal dimensional stability and a high Tg value, and as a result, found that a more rigid linear structure is introduced to and also fluorene is included in a mother nucleus skeleton of a tetracarboxylic acid dianhydride, thereby not only satisfying significantly improved thermal dimensional stability but also increasing transparency. That is, the present inventors confirmed that polyimide derived from a tetracarboxylic acid dianhydride having the structural characteristics as such satisfies a low coefficient of linear thermal expansion, of course, and has excellent transparency and thermal resistance, thereby suggesting the present invention.

Hereinafter, an exemplary embodiment of the present invention will be described in more detail.

The polyimide precursor solution according to an exemplary embodiment of the present invention may include a polyimide precursor derived from a tetracarboxylic acid dianhydride represented by the following Chemical Formula 1 which is a novel tetracarboxylic acid dianhydride.

wherein

Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl;

R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring; and

n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.

By satisfying the structure described above, the polyimide precursor derived from the tetracarboxylic acid dianhydride according to an exemplary embodiment of the present invention may satisfy excellent thermal dimensional stability. That is, thermal resistance may be excellent. In addition, intermolecular packing density may be increased to provide a polyimide film having increased transparency and a decreased yellow index. However, when Q¹ is an oxygen atom in Chemical Formula 1, intramolecular bending occurs and polyimide derived therefrom has a decreased rigid structural characteristic. Thus, the polyimide including the structural characteristic as such (Q¹ is an oxygen atom) has low thermal dimensional stability.

The tetracarboxylic acid dianhydride may be selected from compounds represented by the following Chemical Formulae 2 to 5:

wherein

R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring;

R′ is hydrogen or C1-C4 alkyl; and

n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.

As an example, a tetracarboxylic acid dianhydride having a mother nucleus skeleton derived from spirobifluorene like the compound represented by Chemical Formula 2 may have further improved thermal resistance and thus, also have excellent processability.

As an example, the compound represented by Chemical Formulae 3 to 5 has a further improved yellow index (YI) and an excellent total light transmittance in a region of 380 to 780 nm, thereby providing a polyimide film having both a low coefficient of thermal expansion and high transparency.

The polyimide precursor solution according to an exemplary embodiment of the present invention may include a polyimide precursor derived from the tetracarboxylic acid dianhydride in which in Chemical Formula 1, R¹ and R² may be independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, C1-C10 haloalkyl, or C1-C10 haloalkoxy, and n and m may be independently of each other an integer selected from 0 to 2.

As an example, in Chemical Formulae 2 to 5, R¹ and R² may be independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, C1-C10 haloalkyl, or C1-C10 haloalkoxy, and n and m may be independently of each other an integer selected from 0 to 2.

In the polyimide precursor solution according to an exemplary embodiment of the present invention, in Chemical Formula 1, R¹ and R² may be independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C4 alkyl, C1-C4 alkoxy, C6-C18 aryl, C1-C4 haloalkyl, or C1-C4 haloalkoxy, and n and m may be independently of each other an integer selected from 0 to 2.

As an example, in Chemical Formulae 2 to 5, R¹ and R² may be independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C4 alkyl, C1-C4 alkoxy, C6-C18 aryl, C1-C4 haloalkyl, or C1-C4 haloalkoxy, and n and m may be independently of each other an integer selected from 0 to 2.

The tetracarboxylic acid dianhydride according to an exemplary embodiment of the present invention may include a polyimide precursor derived from the tetracarboxylic acid dianhydride in which in Chemical Formula 1, R¹ and R² may be independently of each other a halogen, nitro, cyano, C1-C4 alkyl, C1-C4 alkoxy, C6-C12 aryl, C1-C4 haloalkyl, or C1-C4 haloalkoxy, and n and m may be independently of each other an integer selected from 0 to 2 and satisfy 0 n+m 2.

As an example, in Chemical Formulae 2 to 5, R¹ and R² may be independently of each other a halogen, nitro, cyano, C1-C4 alkyl, C1-C4 alkoxy, C6-C12 aryl, C1-C4 haloalkyl, or C1-C4 haloalkoxy, and n and m may be independently of each other an integer selected from 0 to 2 and satisfy 0 n+m 2.

As an example, in Chemical Formula 5, R′ may be a hydrogen or C1-C4 alkyl.

As an example, one selected from R¹ and R² may be substituted at position 2 or 7 or at both positions of a fluorene ring. Specifically, one substituent selected from R¹ and R² may be substituted at an indicated position of the following Chemical Formula 1-1. In this case, it may be more effective in mechanical strength and flexibility.

wherein

Q¹ is a single bond, —NR′—, —S—, or —SO₂—, wherein R′ is hydrogen or C1-C4 alkyl; and

R¹ and R² are independently of each other a halogen, nitro, cyano, C1-C4 alkyl, C1-C4 alkoxy, C6-C12 aryl, C1-C4 haloalkyl, or C1-C4 haloalkoxy, and n and m are independently of each other an integer selected from 0 to 2 and satisfy 0 n+m 2.

As an example, in Chemical Formula 1-1, R¹ or R² may be selected from a halogen selected from fluorine, chlorine, bromine, and iodine; alkyl selected from methyl, ethyl, propyl, and butyl; alkoxy selected from methoxy, ethoxy, propoxy, and butoxy; and aryl selected from phenyl and naphthyl.

The tetracarboxylic acid dianhydride may include at least one or two or more selected from the following structures:

wherein Ph is phenyl.

In addition, another exemplary embodiment of the present invention may be a method for preparing the tetracarboxylic acid dianhydride described above.

Specifically, the method for preparing a tetracarboxylic acid dianhydride according to an exemplary embodiment of the present invention may include dehydrating and cyclizing a compound represented by the following Chemical Formula A in the presence of a dehydrating agent:

wherein

Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl;

R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring; and

n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.

As an example, the dehydrating agent may be an acid anhydride.

As an example, the acid anhydride may be selected from acetic anhydrides, phthalic anhydrides, maleic anhydrides, and the like, and preferably, may include acetic anhydrides.

As an example, the dehydrating agent may further include one or two or more selected from pyridine, isoquinoline, triethylamine, and the like.

As an example, the dehydrating agent may be introduced at 2 to 10 mol with respect to 1 mol of the compound represented by Chemical Formula A.

As an example, the step of dehydration and cyclization may be performed at 60 to 130° C. for 2 to 12 hours.

An embodiment for the method for preparing a tetracarboxylic acid dianhydride according to another exemplary embodiment of the present invention may be as shown in the following Reaction Formula 1, but may be variously modified by a common organic synthesis method, of course.

In addition, another exemplary embodiment of the present invention is a use of the tetracarboxylic acid dianhydride described above, and a specific embodiment thereof will be described later.

A first embodiment of the present invention may be a composition including the tetracarboxylic acid dianhydride represented by Chemical Formula 1.

A second embodiment of the present invention may be a polyimide precursor solution including a polyimide precursor derived from the tetracarboxylic acid dianhydride represented by Chemical Formula 1. Specifically, the polyimide precursor may be a polyamic acid obtained by a reaction of a polymerization component including the tetracarboxylic acid dianhydride represented by Chemical Formula 1 and a diamine.

A third embodiment of the present invention may be a polyimide precursor solution including a polyimide precursor derived from the tetracarboxylic acid dianhydride represented by Chemical Formula 1, which may be a polyamic acid further including a tetracarboxylic acid dianhydride known in the art.

A fourth embodiment of the present invention may further include an organic solvent in the first to third embodiments described above.

Specifically, the composition or the solution of the first to fourth embodiments described above may be used for providing a polyimide film. In addition, the composition or the solution of the first to fourth embodiments described above may be used for providing a polyamideimide film.

As described above, according to the present invention, a polyimide film having high transparency and thermal resistance and having excellent thermal dimensional stability due to a substrate of which the stress is not increased even with a heat treatment at a high temperature may be provided. In particular, according to the present invention, even in the case of including no tetracarboxylic acid dianhydride known in the art, a polyimide film having a low coefficient of linear thermal expansion may be provided. In addition, when the tetracarboxylic acid dianhydride known in the art is further included, a polyimide film having a more significantly lowered coefficient of linear thermal expansion may be provided.

The composition or the polyimide precursor solution according to an exemplary embodiment of the present invention may be one or a mixture of two or more selected from ketones such as γ-butyrolactone, 1,3-dimethyl-2-imidazolidinone, methylethylketone, cyclohexanone, cyclopentanone, and 4-hydroxy-4-methyl-2-petanone; aromatic hydrocarbons such as toluene, xylene, and tetramethylbenzene; glycol ethers (cellosolve) such as ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol diethyl ether, and triethylene glycol monoethyl ether; acetates such as ethyl acetate, butyl acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether acetate, and dipropylene glycol monomethyl ether acetate; alcohols such as ethanol, propanol, ethylene glycol, propylene glycol, and carbitol; amides such as N,N-dimethylpropionamide (DMPA), N,N-diethylpropionamide (DEPA), N,N-dimethylacetamide (DMAc), N,N-diethylacetamide, N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), N-methylpyrrolidone (NMP), N-ethylpyrrolidone (NEP), and N,N-dimethylmethoxyacetamide, and the like.

As an example, the organic solvent may be one or a mixture of two or more selected from the amides described above.

As an example, the organic solvent may have a boiling point of 300° C. or lower. Specifically, as an example, the organic solvent may be N,N-diethylformaide (DEF), N,N-diethylacetamide (DEAc), N-ethylpyrrolidone (NEP), N,N-dimethylpropionamide (DMPA), N,N-diethylpropionamide (DEPA), or a combination thereof.

The polyimide precursor solution according to an exemplary embodiment of the present invention may include an aromatic diamine. Specifically, the aromatic diamine according to the present invention may include a unit represented by the following Chemical Formula 6:

wherein

R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl; and

p is an integer of 1 or 2.

As an example, the aromatic diamine may be an aromatic diamine in a linear chain form and specifically, may be selected from compounds represented by the following Chemical Formulae 6-1 and 6-2. Here, the aromatic diamine may be used as one or a mixture of two or more, of course.

wherein

R³ is independently of each other hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl.

As an example, the aromatic diamine may be fluoro-based aromatic diamine including fluoroalkyl.

As an example, the aromatic diamine may be the compound represented by Chemical Formula 6-1 or 6-2, wherein R³ is independently of each other C1-C7 fluoroalkyl.

As an example, the aromatic diamine may be the compound represented by Chemical Formula 6-1 or 6-2, wherein R³ is independently of each other C1-C3 fluoroalkyl.

The polyimide precursor solution according to an exemplary embodiment of the present invention may include a tetracarboxylic acid dianhydride having a mother nucleus skeleton derived from spirobifluorene like the compound represented by Chemical Formula 2, in terms of imparting excellent processability with further improved thermal resistance.

The polyimide precursor solution according to an exemplary embodiment of the present invention may include one or more tetracarboxylic acid dianhydrides selected from the compounds represented by Chemical Formulae 3 to 5, in terms of implementing further improved yellow index (YI) and an excellent total light transmittance in a region of 380 to 780 nm.

The polyimide precursor solution according to an exemplary embodiment of the present invention may include the tetracarboxylic acid dianhydride of Chemical Formula 1-1 wherein R¹ or R² is C1-C4 alkyl, in terms of implementing further improved mechanical strength and flexibility.

The polyimide precursor solution according to an exemplary embodiment of the present invention may further include a tetracarboxylic acid dianhydride known in the art selected from compounds represented by the following Chemical Formulae 7 and 8 as a polymerization component:

wherein

Q³ and Q³ are a single bond, —O—, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, phenylene, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl.

The polyimide precursor solution according to an exemplary embodiment of the present invention may include the tetracarboxylic acid dianhydride represented by Chemical Formula 1 and diamine as a polymerization component, as described above. Specifically, the polymerization component may include the tetracarboxylic acid dianhydride represented by Chemical Formula 1 in a range of 0.9 to 1.1 based on 1 mol of the diamine.

In addition, when the polyimide precursor solution according to an exemplary embodiment of the present invention further includes the tetracarboxylic acid dianhydride known in the art as described above, a total content of the tetracarboxylic acid dianhydride represented by Chemical Formula 1 and the tetracarboxylic acid dianhydride known in the art and a content of the diamine may be included at a mole ratio of 1:0.99 to 0.99:1, specifically 1:0.98 to 0.98:1.

In addition, when the polyimide precursor solution according to an exemplary embodiment of the present invention further includes the tetracarboxylic acid dianhydride known in the art as described above, the tetracarboxylic acid dianhydride represented by Chemical Formula 1 may be included at 10 to 99 mol %, based on the total content of the tetracarboxylic acid dianhydride represented by Chemical Formula 1 and the tetracarboxylic acid dianhydride known in the art. Otherwise, it may be included at 10 to 90 mol %, 20 to 50 mol %, or 20 to 30 mol %.

Specifically, the polyimide precursor solution according to an exemplary embodiment of the present invention may include a polyimide precursor including a repeating unit represented by the following Chemical Formula a which is a polymerization product of the polymerization component described above, that is, a polyamic acid:

wherein

Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl;

R^(a) and R^(b) are independently of each other hydrogen or C1-C10 alkyl;

R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring;

R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl;

p is an integer of 1 or 2; and

n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.

As an example, the polyimide precursor solution according to an exemplary embodiment of the present invention may have a solid content including the polyimide precursor including the repeating unit represented by Chemical Formula a of 10 to 40 wt %, 10 to 30 wt %, 10 to 20 wt %, or 10 to 13 wt %, based on the total weight. Here, the solid content may be the content of the polyamic acid and the residual amount may be the amount of the organic solvent.

As an example, the polyimide precursor solution according to an exemplary embodiment of the present invention may have a viscosity satisfying 2,000 to 10,000 cps. The viscosity may satisfy specifically 8,000 cps or less, more specifically 7,000 cps or less. When the viscosity rang as such is satisfied, deformation efficiency when processing a polyimide film is excellent to provide an advantage in processing. Thus, a more uniform surface may be implemented, which is preferred. Here, the viscosity refers to a value measured by piling up a sample at room temperature (25° C.) using a Brookfield RVDV-III viscometer spindle No. 52 and performing a stabilization operation for 2 minutes when a torque value is 80%.

The polyimide precursor solution according to an exemplary embodiment of the present invention may be obtained by polymerizing the polymerization component described above to prepare a polyamic acid including the repeating unit represented by Chemical Formula a and then performing imidization. As an example, the imidization may be performed by a chemical imidization method or a thermal imidization method.

The imidization according to an exemplary embodiment of the present invention may be performed by the thermal imidization method. By the method as such, uniform mechanical physical properties may be imparted to the entire film when imidized by heat at a high temperature. Specifically, the polyimide film according to an exemplary embodiment of the present invention may be prepared by a preparation method including applying and coating the polyimide precursor solution described above and then performing a heat treatment.

As an example, the heat treatment may be performed at 500° C. or lower.

As an example, the heat treatment may include a first heat treatment performed at 100° C. or lower; a second heat treatment performed at higher than 100° C. and 300° C. or lower; and a third heat treatment performed at higher than 300° C. and 500° C. or lower, but is not limited thereto.

As an example, the substrate may be a glass substrate, a metal substrate, or a plastic substrate, without particular limitation. Among them, the substrate may be the glass substrate which has excellent thermal and chemical stability during imidization and a curing process for the polyimide precursor solution and may be easily separated without damage to a polyimide film formed after curing.

As an example, a method for application and coating is not particularly limited, but for example, any one or more methods selected from a spin coating method, a dipping method, a spraying method, a die coating method, a bar coating method, a roll coating method, a meniscus method, a flexography method, a screen printing method, a bead coating method, an air knife coating method, a reverse roll coating method, a blade coating method, a casting coating method, a gravure coating method, and the like may be used.

As an example, after the heat treatment step, a drying step and a step of separation from the substrate may be further included.

As an example, a molecular weight of the polyamic acid including the repeating unit represented by Chemical Formula a is not particularly limited, but, as an example, when the weight average molecular weight is in a range of 20,000 to 150,000 g/mol, better physical properties may be obtained.

In addition, the polyimide precursor solution according to an exemplary embodiment of the present invention may further include an additive such as a leveling agent, a flame retardant, an adhesion improver, inorganic particles, an antioxidant, a UV inhibitor, and a plasticizer.

Specifically, the polyimide film prepared from the polyimide precursor solution according to an exemplary embodiment of the present invention, that is, a polyimide may include a repeating unit represented by the following Chemical Formula b:

wherein

Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl;

R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring;

R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl;

p is an integer of 1 or 2; and

n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.

In addition, the polyimide film according to an exemplary embodiment of the present invention may further include a repeating unit represented by the following Chemical Formula c or d:

wherein

R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl; and

p is an integer of 1 or 2.

As described above, according to an exemplary embodiment of the present invention, as the repeating unit derived from the tetracarboxylic acid dianhydride represented by Chemical Formula 1 is included, a polyimide film having optical properties, thermal resistance, mechanical strength, and flexibility which are all excellent may be provided. Thus, the polyimide film may be used in various fields such as an element substrate, a display cover substrate, an optical film, an integrated circuit (IC) package, an electrodeposition film, a multilayer flexible printed circuit (FRC), a tape, a touch panel, and an optical disc protective film.

The polyimide film according to an exemplary embodiment of the present invention, that is, the polyimide may have a weight average molecular weight of 10,000 to 200,000 g/mol, 20,000 to 100,000 g/mol, or 30,000 to 100,000 g/mol. In addition, the polyimide according to the present invention may have a molecular weight distribution (Mw/Mn) satisfying a range of 1.1 to 2.5. When the weight average molecular weight and the molecular weight distribution of the polyimide are satisfied, it is advantageous for the characteristics of the polyimide film such as optical properties, thermal resistance, mechanical strength, and flexibility.

The polyimide film according to an exemplary embodiment of the present invention may have a thickness of 5 to 15 μm.

The polyimide film according to an exemplary embodiment of the present invention may have excellent thermal resistance properties depending on a temperature change. Specifically, as a result of measuring a thermal expansion change pattern when subjected to a primary heating process at a heating rate of 5° C./min in a temperature range of 100° C. to 450° C. in the thickness range described above and then cooled at a cooling rate of 4° C./min in a temperature range of 400° C. to 100° C. using TMA (TMA450 available from TMA), the polyimide film may satisfy 50 ppm/° C. or less. Specifically, the polyimide film may satisfy the coefficient of thermal expansion (CTE) of 45 ppm/° C. or less, more specifically in a range of −15 to 45 ppm/° C.

The polyimide film according to an exemplary embodiment of the present invention may satisfy a haze in accordance with ASTM D1003 in a range of 2 or less, specifically 1 or less, more specifically 0.5 or less, and most specifically 0.01 to 0.3, in the thickness range described above. Since the haze value as such is satisfied, a polyimide film having improved transparency may be provided.

In addition, the polyimide film according to an exemplary embodiment of the present invention has an excellent light transmittance and an excellent yellow index, thereby representing significantly improved transparency and optical properties. Specifically, the polyimide film may have YI in accordance with ASTM E313 of 15 or less and a total light transmittance in a region of 380 to 780 nm in accordance with ASTM D1746 of 80% or more, more specifically YI of 13 or less and a total light transmittance of 85% or more, and most specifically YI of 11 or less and a total light transmittance of 87% to 99%.

The polyimide film according to an exemplary embodiment of the present invention may have a modulus in accordance with ASTM D882 of 5.0 or more and an elongation of 15% or more, in the thickness range described above. Specifically, the polyimide film may have a modulus of 5.5 or more and an elongation of 15% or more, and more specifically a modulus of 6.0 or more and an elongation of 15% or more. When these characteristics are satisfied, the polyimide film may have excellent rigidity and secure further sufficient flexibility, thereby having a flexible property from external impact.

The polyimide film according to an exemplary embodiment of the present invention satisfying all of the physical properties described above at the same time may be preferred, but the present invention is not limited thereto.

The polyimide film according to an exemplary embodiment of the present invention may satisfy excellent optical properties, thermal resistance, mechanical strength, and flexibility at the same time by a rigid structure derived from the tetracarboxylic acid dianhydride represented by Chemical Formula 1. In particular, since the polyimide film may represent excellent thermal resistance to thermal shrinkage behavior which may occur in a process at a high temperature and also may represent excellent colorless transparent optical properties, the polyimide film may be used in various fields such as an element substrate, a display substrate, an optical film, an integrated circuit (IC) package, an electrodeposition film (adhesive film), a multilayer flexible printed circuit (FRC), a tape, a touch panel, and an optical disc protective film.

The polyimide film according to an exemplary embodiment of the present invention may be used in the form of being included as two or more layers.

In addition, another exemplary embodiment of the present invention may be a photoelectric device and a flexible display including the polyimide film or a multilayer structure in the form in which the polyimide films are included, as a flexible substrate.

As an example, the photoelectric device may be an optical component, a switch, an optical modulator, and also, is appropriate as a high thermal resistant substrate material requiring a micropattern formation characteristic.

As an example, the flexible display may be appropriate for a liquid crystal display device (LCD), an organic light emitting diode (OLED), and the like, and particularly, may be appropriate for an OLED device using a low temperature polysilicon (LTPS) process requiring a process at a high temperature, but is not limited thereto.

Hereinafter, the present invention will be described by the specific Examples and Comparative Examples of the present invention. The following Examples are for describing the technical idea of the present invention, and it is apparent to a person skilled in the art that the present invention is not limited thereto.

(Evaluation Method)

1. Coefficient of linear thermal expansion (CTE) and glass transition temperature (Tg)

The coefficient of linear thermal expansion was measured in accordance with a TMA-method using TMA (available from TA Instrument, Discovery 450). A specimen size was 5 mm×20 mm, a loading was 0.02 N, and a heating rate was 5° C./min. The CTE value was measured in a heating region at a temperature in a range of 100° C. to 450° C.

The Tg value was measured as a TMA graph inflection point in a heating region of 100° C. to 450° C.

2. Haze

The haze was measured using a spectrophotometer (available from Nippon Denshoku, COH-400), on a polyimide film having a thickness of 50 μm, in accordance with the standard of ASTM D1003. The unit was %.

3. Yellow Index (YI)

The yellow index was measured using a colorimeter (available from HunterLab, ColorQuest XE), on a polyimide film having a thickness of 10 μm, in accordance with the standard of ASTM E313.

4. Total Light Transmittance

The total light transmittance was measured in the entire wavelength range of 380 to 780 nm using a spectrophotometer (available from SHIMADZU, MPC-3100) on a polyimide film having a thickness of 10 μm, in accordance with the standard of ASTM D1746. The unit was %.

5. Modulus and Elongation

In accordance with ASTM D882, the elongation was measured using UTM 3365 available from Instron, under the condition of pulling a polyimide film having a thickness of 10 μm, a length of 40 mm and a width of 5 mm at 10 mm/min at 25° C. The unit of the modulus was GPa and the unit of the elongation was %.

6. Thickness

PAA was coated on a 0.5 T glass and cured to obtain a substrate, and the thickness of the substrate was measured using a film thickness measuring instrument (Alpha step D500). The unit was μm.

7. Viscosity

The viscosity may refer to a value measured by piling up a sample at room temperature (25° C.) using a Brookfield RVDV-III viscometer spindle No. 52, allowing the sample to stand for 2 minutes when a torque value is 80%, and then stabilizing the sample. The unit was cps.

8. C.R. Test (Chemical Resistance Test)

Polyimide film curing conditions: Multi-step (80° C./30 min, 220° C./30 min, and 450° C./60 min)

∘: no film deformation, Δ: occurrence of partial film deformation, x: occurrence of film deformation.

9. Weight Average Molecular Weight

Measurement was performed by dissolving a film in a DMAc eluent containing 0.05 M LiBr. For GPC, Waters GPC system, Waters 1515 isocratic HPLC Pump, and Waters 2414 Reflective Index detector were used, for a column, Olexis, Polypore and a mixed D column were connected and polymethylmethacrylate (PMMA STD) was used as a standard material, and analysis was performed at 35° C. at a flow rate of 1 mL/min.

Preparation Example 1

Step 1. Material A

3,4-Dimethylphenylboronic acid (7.95 g, 53.0 mmol), 1,2-dibromo-4,5-dimethylbenzene (14 g, 53.0 mmol), triphenylphosphine (0.42 g, 1.59 mmol), and K₂CO₃ (21.9 g, 159 mmol) were added to a mixed solution of 100 ml of degassed water and 70 ml of tetrahydrofuran (THF) in a nitrogen environment. The temperature was raised to 80° C., Pd(PPh₃)₄ (0.61 g, 0.53 mmol) was added, and stirring was performed for 30 hours. The temperature was lowered to room temperature, the organic solvent was removed by distillation under reduced pressure, 100 ml of dichloromethane (DCM) and 50 ml of water were added, an organic layer was washed with water, and moisture was removed using anhydrous MgSO₄. After MgSO₄ filtering, the solvent was removed and the material was separated with column chromatography to obtain Material A with a yield of 75%.

HRMS (EI, m/z): [M+] calculated for C16H13Br, 288.05; found, 269.16.

¹H-NMR (ppm, CDCl₃): 7.44 (1H, s), 7.15-7.20 (3H, m), 7.10 (1H, s), 2.33 (6H, s), 2.29 (3H, s), 2.25 (3H, s).

Step 2. Material B

Material A (5.65 g, 19.5 mmol) was dissolved in 80 ml of anhydrous tetrahydrofuran (THF) in a nitrogen environment and n-butyl lithium (9.37 ml of a solution 2.5 M in hexane, 23.4 mmol) was slowly added at −78° C. for 1 hour. Anhydrous CO₂ was blown in the solution for 4 hours, 60 ml of water was added to terminate the reaction, and the organic solvent was removed by distillation under reduced pressure. 100 ml of water and 50 ml of DCM were added, an aqueous layer was washed with DCM, a 0.1 N HCl solution was added to the aqueous layer to bring pH to 4, and then a precipitated organic material was extracted with ether. Anhydrous MgSO₄ was used to remove moisture and filtered, the solvent was removed, and the material was separated with column chromatography to obtain Material B with a yield of 65%.

HRMS (EI, m/z): [M+] calculated for C17H1802, 254.13; found, 255.32.

¹H-NMR (ppm, CDCl₃): 7.75 (1H, s), 7.12-7.15 (3H, m), 7.07 (1H, s), 2.34 (s, 6H), 2.31 (6H, s).

Step 3. Material C

Material B (4.73 g, 18.6 mmol) was dissolved in 50 g of MeSO₃H in a nitrogen environment and stirring was performed at 50° C. for 18 hours. The reactant was poured into 500 ml of water at 0° C., a produced solid was filtered, and recrystallization was performed using methanol (MeOH) to obtain Material C with a yield of 85%.

HRMS (EI, m/z): [M+] calculated for C17H160, 236.12; found, 237.25.

¹H-NMR (ppm, CDCl₃) 7.24 (2H, s), 2.33 (6H, s), 2.28 (6H, s).

Step 4. Material D

2-Bromo-1,1′-biphenyl (4.0 g, 17.16 mmol) was dissolved 80 ml of anhydrous tetrahydrofuran (THF) in a nitrogen environment and n-butyl lithium (7.5 ml of a solution 2.5 M in hexane, 18.7 mmol) was slowly added at −78° C. for 10 minutes. After stirring for 1 hour, Material C (3.69 g, 15.6 mmol) was added to a reactant and stirring was performed for 12 hours while raising a temperature to room temperature. 80 ml of water was added, pressure was reduced to remove a solvent, and 100 ml of dichloromethane (DCM) was added to extract an organic material. Anhydrous MgSO₄ was used to remove moisture and filtered to remove the solvent, and then the material was added to 50 ml of acetic acid at 0° C. 1 ml of 35 wt % HCl was added, heating to reflux was performed for 4 hours, and then stirring was performed at room temperature for 1 hour. The reactant was poured into 200 ml of ice water, a produced solid was filtered, and methanol (MeOH) was used to perform precipitation and stirring, thereby obtaining Material D with a yield of 87%.

HRMS (EI, m/z): [M+] calculated for C29H24, 372.19; found, 373.05.

¹H-NMR (ppm, CDCl₃): 7.84-7.91 (4H, m), 7.59 (2H, s), 6.73-6.80 (4H, m), 6.49 (2H, s), 2.35 (6H, s), 2.11 (6H, s).

Step 5. Material E

Material D (3.95 g, 10.6 mmol) was dissolved in a mixed solvent of 50 ml of pyridine (Py) and 50 ml of water, and KMnO₄ (33.5 g, 212 mmol) dissolved in 100 ml of water was slowly added for 4 hours. After heating to reflux for 6 hours, filtration was performed to remove a solid material, and the temperature was lowered to room temperature to remove a solvent. A 0.1 N HCl solution was added to an aqueous layer to bring pH to 4 and a precipitated solid material was filtered and dried to obtain Material E with a yield of 80%.

HRMS (EI, m/z): [M+] calculated for C29H1608, 492.08; found, 493.00.

¹H-NMR (ppm, D₂O): 7.97 (8H, s), 7.91 (4H, s).

Step 6. Material F

Material E (3.80 g, 7.72 mmol) was dissolved in acetic acid anhydride (100 ml) in a nitrogen environment, heating to reflux was performed for 6 hours, and then the temperature was lowered to room temperature. The produced solid was filtered and washed using acetic acid anhydride, and then Material F was obtained with a yield of 90%.

HRMS (EI, m/z): [M+] calculated for C29H1206, 456.06; found, 457.11.

Preparation Example 2

Step 1. Material G

Material A (5.0 g, 17.47 mmol) was dissolved in 100 ml of anhydrous tetrahydrofuran (THF), the temperature was lowered to −78° C., and then n-butyl lithium (7.6 ml of a solution 2.5 M in hexane, 18.8 mmol) was slowly added for 10 minutes. After stirring for 1 hour, 9H-thioxanthen-9-one (3.375 g, 15.9 mmol) was added to a reactant, and stirring was performed for 12 hours while the temperature was raised to room temperature. 100 ml of water was added, pressure was reduced to remove a solvent, and 100 ml of dichloromethane (DCM) was added to extract an organic material. Anhydrous MgSO₄ was used to remove moisture and filtered to remove the solvent, and then the material was added to 50 ml of acetic acid at 0° C. 1 ml of 35 wt % HCl was added, heating to reflux was performed for 4 hours, and then stirring was performed at room temperature for 1 hour. The reactant was poured into 200 ml of water, a produced solid was filtered, and MeOH was used to perform precipitation and stirring, thereby obtaining Material G with a yield of 88%.

HRMS (EI, m/z): [M+] calculated for C29H24S, 404.16; found, 405.15.

Step 2. Material H

Material G (4.6 g, 10.88 mmol) was dissolved in 100 ml of acetic acid, and 9 ml of 30 wt % H₂O₂ was slowly added for 30 minutes. The temperature was raised to 100° C., stirring was performed for 8 hours, and the temperature was lowered to room temperature. The produced solid material was filtered, was further washed with 20 ml acetic acid and 20 ml of heptane, and was dried, thereby obtaining Material H with a yield of 89%.

HRMS (EI, m/z): [M+] calculated for C29H24S, 404.16; found, 405.15.

Step 3. Material I

Material H (4.0 g, 9.89 mmol) was dissolved in a mixed solvent of 50 ml of pyridine (Py) and 50 ml of water, and KMnO₄ (31.6 g, 200 mmol) dissolved in 100 ml of water was slowly added for 4 hours. After heating to reflux for 6 hours, filtration was performed to remove a solid material, and the temperature was lowered to room temperature to remove a solvent. A 0.1 N HCl solution was added to an aqueous layer to bring pH to 4 and a precipitated solid material was filtered and dried to obtain Material I with a yield of 85%.

HRMS (EI, m/z) [M+] calculated for C29H16010S, 556.05; found, 557.04.

Step 4. Material J

Material I (3.6 g, 6.47 mmol) was dissolved in acetic acid anhydride (80 ml) in a nitrogen environment, heating to reflux was performed for 6 hours, and then the temperature was lowered to room temperature. The produced solid was filtered, washed using acetic acid anhydride, and dried to obtain Material J with a yield of 90%.

HRMS (EI, m/z): [M+] calculated for C29H1206, 556.05; found, 557.03.

Preparation Example 3

Step 1. Material K

2-Bromo-4′-tert-butyl-1,1′-biphenyl (5.0 g, 17.29 mmol) was 80 ml of anhydrous tetrahydrofuran (THF) in a nitrogen environment, the temperature was lowered to −78° C., and then n-butyl lithium (7.6 ml of a solution 2.5 M in hexane, 18.8 mmol) was slowly added for 10 minutes. After stirring for 1 hour, Material C (3.73 g, 15.8 mmol) was added to a reactant and stirring was performed for 12 hours while raising a temperature to room temperature. 80 ml of water was added, pressure was reduced to remove a solvent, and 100 ml of dichloromethane (DCM) was added to extract an organic material. Anhydrous MgSO₄ was used to remove moisture and filtered to remove the solvent, and then the material was added to 50 ml of acetic acid at 0° C. 1 ml of 35 wt % HCl was added, heating to reflux was performed for 4 hours, and then stirring was performed at room temperature for 1 hour. The reactant was poured into 200 ml of water at 0° C., a produced solid was filtered, and methanol (MeOH) was used to perform precipitation and stirring, thereby obtaining Material K with a yield of 85%.

HRMS (EI, m/z): [M+] calculated for C33H32, 428.25; found, 428.20.

Step 2. Material L

Material K (3.95 g, 9.22 mmol) was dissolved in a mixed solvent of 50 ml of pyridine (Py) and 50 ml of water, and KMnO₄ (31.0 g, 190 mmol) dissolved in 100 ml of water was slowly added for 4 hours. After heating to reflux for 6 hours, filtration was performed to remove a solid material, and the temperature was lowered to room temperature to remove a solvent. A 0.1 N HCl solution was added to an aqueous layer to bring pH to 4 and a precipitated solid material was filtered and dried to obtain Material L with a yield of 81%.

HRMS (EI, m/z): [M+] calculated for C33H2408, 548.15; found, 548.12.

Step 3. Material M

Material L (3.00 g, 5.47 mmol) was dissolved in acetic anhydride (80 ml) in a nitrogen environment, heating to reflux was performed for 6 hours, and then the temperature was lowered to room temperature. The produced solid was filtered and washed using acetic acid anhydride, and then Material M was obtained with a yield of 90%.

HRMS (EI, m/z): [M+] calculated for C33H20O6, 512.13; found, 513.12.

Example 1

TFMB (0.999)/Example 1 (1.0), unit: mole ratio

A stirrer in which a nitrogen stream flows was filled with 176 g of N,N-dimethylpropionamide (DMPA), and then 14.03 g of 2,2′-bis(trifluoromethyl)-4,4′-biphenyl diamine (TFMB) was dissolved in the state in which the temperature of the reactor was maintained at 25° C. 20 g of a novel monomer 1 (Preparation Example 1, Material F) was added thereto at the same temperature and stirring was performed for a certain period of time while dissolving the monomer. Thereafter, DMPA was added so that a solid concentration was 13 wt %, thereby preparing a polyimide precursor solution 1. The polyimide precursor solution 1 had a viscosity of 4,500 cp.

Example 2

TFMB (0.999)/Example 1 (0.2)/PMDA (0.8), unit: mole ratio

A stirrer in which a nitrogen stream flows was filled with 175 g of N,N-dimethylpropionamide (DMPA), and then 18.33 g of 2,2′-bis(trifluoromethyl)-4,4′-biphenyl diamine (TFMB) was dissolved in the state in which the temperature of the reactor was maintained at 25° C. 5.22 g of a novel monomer 1 (Preparation Example 1, Material F) and 10 g of pyromellitic dianhydride (PMDA) were added thereto at the same temperature and stirring was performed for a certain period of time while dissolving the monomer. Thereafter, DMPA was added so that the solid concentration of the polyimide precursor solution was 13 wt %, thereby preparing a polyimide precursor solution 2. The polyimide precursor solution 2 had a viscosity of 5,200 cp.

Example 3

TFMB (0.999)/Example 2 (1.0), unit: mole ratio

A stirrer in which a nitrogen stream flows was filled with 169 g of N,N-dimethylpropionamide (DMPA), and then 12.29 g of 2,2′-bis(trifluoromethyl)-4,4′-biphenyl diamine (TFMB) was dissolved in the state in which the temperature of the reactor was maintained at 25° C. 20 g of a novel monomer 2 (Preparation Example 2, Material J) was added thereto at the same temperature and stirring was performed for a certain period of time while dissolving the monomer. Thereafter, DMPA was added so that a solid concentration was 13 wt %, thereby preparing a polyimide precursor solution 3. The polyimide precursor solution 3 had a viscosity of 4,200 cp.

Example 4

TFMB (0.999)/Example 3 (1.0), unit: mole ratio

A stirrer in which a nitrogen stream flows was filled with 170 g of N,N-dimethylpropionamide (DMPA), and then 12.48 g of 2,2′-bis(trifluoromethyl)-4,4′-biphenyl diamine (TFMB) was dissolved in the state in which the temperature of the reactor was maintained at 25° C. 20 g of a novel monomer 3 (Preparation Example 3, Material M) was added thereto at the same temperature and stirring was performed for a certain period of time while dissolving the monomer. Thereafter, DMPA was added so that a solid concentration was 13 wt %, thereby preparing a polyimide precursor solution 4. The polyimide precursor solution 4 had a viscosity of 4,300 cp.

Example 5

TFMB (0.999)/Example 2 (0.2)/PMDA (0.8), unit: mole ratio

A stirrer in which a nitrogen stream flows was filled with 180 g of N,N-dimethylpropionamide (DMPA), and then 18.33 g of 2,2′-bis(trifluoromethyl)-4,4′-biphenyl diamine (TFMB) was dissolved in the state in which the temperature of the reactor was maintained at 25° C. 5.96 g of a novel monomer 2 (Preparation Example 2, Material J) and 10 g of pyromellitic dianhydride (PMDA) were added thereto at the same temperature and stirring was performed for a certain period of time while dissolving the monomer. Thereafter, DMPA was added so that a solid concentration was 13 wt %, thereby preparing a polyimide precursor solution 5. The polyimide precursor solution 5 had a viscosity of 5,200 cp.

Example 6

TFMB (0.999)/Example 3 (0.2)/PMDA (0.8), unit: mole ratio

A stirrer in which a nitrogen stream flows was filled with 179 g of N,N-dimethylpropionamide (DMPA), and then 18.33 g of 2,2′-bis(trifluoromethyl)-4,4′-biphenyl diamine (TFMB) was dissolved in the state in which the temperature of the reactor was maintained at 25° C. 5.87 g of a novel monomer 3 (Preparation Example 3, Material M) and 10 g of pyromellitic dianhydride (PMDA) were added thereto at the same temperature and stirring was performed for a certain period of time while dissolving the monomer. Thereafter, DMPA was added so that a solid concentration was 13 wt %, thereby preparing a polyimide precursor solution 6. The polyimide precursor solution 6 had a viscosity of 5,200 cp.

Comparative Example 1

TFMB (0.999)/BPAF (1.0), unit: mole ratio

A stirrer in which a nitrogen stream flows was filled with 173 g of N,N-dimethylpropionamide (DMPA), and then 13.96 g of 2,2′-bis(trifluoromethyl)-4,4′-biphenyl diamine (TFMB) was dissolved in the state in which the temperature of the reactor was maintained at 25° C. 20 g of 9,9′-bis(3,4-dicaroxyphenyl)fluorene dianhydride (BPAF) was added thereto at the same temperature and stirring was performed while dissolving the compound for a certain period of time. Thereafter, DMPA was added so that a solid concentration was 13 wt %, thereby preparing a polyimide precursor solution A. The polyimide precursor solution A had a viscosity of 4,200 cp.

Comparative Example 2

TFMB (0.999)/BPAF (0.2)/PMDA (0.8), unit: mole ratio

A stirrer in which a nitrogen stream flows was filled with 175 g of N,N-dimethylpropionamide (DMPA), and then 18.33 g of 2,2′-bis(trifluoromethyl)-4,4′-biphenyl diamine (TFMB) was dissolved in the state in which the temperature of the reactor was maintained at 25° C. 5.25 g of 9,9′-bis(3,4-dicaroxyphenyl)fluorene dianhydride (BPAF) and 10 g of pyromellitic dianhydride (PMDA) were added thereto at the same temperature and stirring was performed while dissolving the compound for a certain period of time. Thereafter, DMPA was added so that a solid concentration was 13 wt %, thereby preparing a polyimide precursor solution B. The polyimide precursor solution B had a viscosity of 4,600 cp.

(Preparation of Polyimide Film)

Each of the polyimide precursor solutions of Examples 1 to 6 and Comparative Examples 1 and 2 was spin-coated on a glass substrate. The glass substrate on which the polyimide precursor solution was coated was placed in an oven, heated at a rate of 4° C./min, and maintained at 80° C. for 30 minutes, at 220° C. for 30 minutes, and at 450° C. for 1 hour to proceed with a curing process. After completing the curing process, the glass substrate was immersed in water to form a film on the glass substrate, and the film was detached and dried in an oven at 100° C. to prepare a polyimide film.

The physical properties of the polyimide film prepared by the above method were measured by the above evaluation methods, and are shown in the following Table 1:

TABLE 1 Example Example Example Example Example Example Comparative Comparative Item unit 1 2 3 4 5 6 Example 1 Example 2 Thickness um 10 10 10 10 10 10 10 10 Sol. Con. wt % 10.5 10.3 11.8 11.2 10.2 10.3 13.5 10.5 Viscosity cps 4500 5200 4200 4300 5200 5200 4200 4600 Tg ° C. N.D. N.D. N.D. 450 N.D. N.D. 400 N.D. Td1% ° C. 560 558 553 515 535 510 558 559 CTE ppm/° 35 6.5 39 43 7.6 8.3 55 8.9 100~450° C. C. Total light % 88 87 90 88 88 88 88 87 transmittance YI — 7.5 10.8 7.2 7.5 7.4 7.8 6.5 10.3 Haze % 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.2 Modulus GPa 6.8 7.5 6.5 6.1 7.2 7.1 5.9 6.8 Elongation % 20 25 27 15 29 23 13 15 C.R. test — ∘ ∘ ∘ Δ ∘ ∘ Δ ∘ (Stripper & Developer)

As shown in the above Table 1, in Examples 1, 3, and 4 which were polyimide films derived from the tetracarboxylic acid dianhydride according to the present invention, a coefficient of thermal expansion was 35-43 ppm/° C., a haze was 0.1 to 0.2, a yellow index (YI) was 7.2 to 7.5, a total light transmittance was 88 to 90%, a modulus was 6.1 to 6.8, and an elongation was 15 to 27%, and the characteristics for each item were measured at a similar level.

In addition, in Examples 2, 5, and 6 which were polyimide films further including a repeating unit derived from pyromellitic dianhydride (PMDA), a coefficient of thermal expansion was 6.5 to 8.3 ppm/° C., a haze was 0.1, a yellow index (YI) was 7.4 to 10.8, a total light transmittance was 87 to 88%, a modulus was 7.1 to 7.5, and an elongation was 23 to 29%, and the characteristics for each item were measured at a similar level.

The effects as such were thermal resistance, optical properties, and mechanical properties which were significantly improved as compared with Comparative Example 1 or 2.

According to an exemplary embodiment of the present invention, a polyimide film having high transparency and thermal resistance and having excellent thermal dimensional stability due to a substrate of which the stress is not increased even with a heat treatment at a high temperature may be provided. In addition, the polyimide film according to an exemplary embodiment of the present invention may be optically very good with decreased optical anisotropy and implement uniform transmittance to total light.

According to an exemplary embodiment of the present invention, a colorless and transparent polyimide film may be provided. The polyimide film according to an exemplary embodiment of the present invention has excellent thermal resistance, mechanical strength, and flexibility, and may be useful in various fields such as a device substrate, a flexible display substrate, an optical film, an integrated circuit (IC) package, an adhesive film, a multilayer flexible printed circuit (FPC), a tape, a touch panel, and a protective film for an optical disc.

Specifically, the polyimide film according to an exemplary embodiment of the present invention i) may satisfy a coefficient of thermal expansion (CTE) of 50 ppm/° C. or less at 100 to 450° C., ii) may satisfy YI in accordance with ASTM E313 of 15 or less, a haze in accordance with ASTM D1003 of 2 or less, and an average light transmittance in a region of 380 to 780 nm in accordance with ASTM D1746 of 80% or more, iii) may satisfy a modulus in accordance with ASTM D882 of 8.0 or less and an elongation of 15% or more, or may satisfy these properties at the same time.

Hereinabove, although the present invention has been described by specified matters and specific exemplary embodiments, they have been provided only for assisting in the entire understanding of the present invention. Therefore, the present invention is not by the specific matters limited to the exemplary embodiments, and various modifications and changes may be made by those skilled in the art to which the present invention pertains from this description.

Therefore, the spirit of the present invention should not be limited to the above-described exemplary embodiments, and the following claims as well as all modified equally or equivalently to the claims are intended to fall within the scope and spirit of the invention. 

What is claimed is:
 1. A polyimide precursor solution comprising: a polyimide precursor derived from a tetracarboxylic acid dianhydride represented by the following Chemical Formula 1:

wherein Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl; R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring; and n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.
 2. The polyimide precursor solution of claim 1, wherein the tetracarboxylic acid dianhydride is selected from compounds represented by the following Chemical Formulae 2 to 5:

wherein R¹, R², R′, n, and m are as defined in Chemical Formula 1 of claim
 1. 3. The polyimide precursor solution of claim 1, wherein the polyimide precursor solution includes: a polymerization component including the tetracarboxylic acid dianhydride represented by Chemical Formula 1 and a diamine; and an organic solvent.
 4. The polyimide precursor solution of claim 3, wherein the diamine includes a unit represented by the following Chemical Formula 6:

wherein R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl; and p is an integer of 1 or
 2. 5. The polyimide precursor solution of claim 3, wherein the organic solvent is selected from amides.
 6. The polyimide precursor solution of claim 5, wherein the organic solvent is N,N-diethylacetamide, N,N-diethylformamide, N-ethylpyrrolidone, N,N-dimethylpropionamide, N,N-diethylpropionamide, or a combination thereof.
 7. The polyimide precursor solution of claim 3, wherein a tetracarboxylic acid dianhydride selected from compounds represented by the following Chemical Formulae 7 and 8 is further included as the polymerization component:

wherein Q² and Q³ are a single bond, —O—, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, phenylene, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl.
 8. A polyimide precursor derived from a tetracarboxylic acid dianhydride represented by the following Chemical Formula 1 and a diamine:

wherein Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl; R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring; and n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.
 9. The polyimide precursor of claim 8, wherein the polyimide precursor includes a repeating unit represented by the following Chemical Formula a:

wherein Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl; R^(a) and R^(b) are independently of each other hydrogen or C1-C10 alkyl; R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring; R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl; p is an integer of 1 or 2; and n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.
 10. The polyimide precursor of claim 9, wherein the polyimide precursor includes 10 to 100 mol % of the repeating unit represented by Chemical Formula a, based on total repeating units.
 11. A polyimide film comprising a repeating unit represented by the following Chemical Formula b:

wherein Q¹ is a single bond, —C(═O)—, —C(═O)O—, —C(═O)NH—, —NR′—, —S—, —SO₂—, —CH₂—, or a combination thereof, wherein R′ is hydrogen or C1-C10 alkyl; R¹ and R² are independently of each other a halogen, hydroxy, thiol, nitro, cyano, C1-C10 alkyl, C1-C10 alkoxy, C6-C20 aryl, or a combination thereof, or may be connected to an adjacent substituent to form a ring; R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl; p is an integer of 1 or 2; and n and m are independently of each other an integer selected from 0 to 4, and when n and m are an integer of 2 or more, R¹ and R² may be the same as or different from each other.
 12. The polyimide film of claim 11, wherein the polyimide film further includes a repeating unit represented by the following Chemical Formula c or d:

wherein R³ is hydrogen, C1-C10 alkyl, or C1-C10 fluoroalkyl; and p is an integer of 1 or
 2. 13. The polyimide film of claim 11, wherein the polyimide film has a coefficient of thermal expansion (CTE) of 50 ppm/° C. or less at 100 to 450° C.
 14. The polyimide film of claim 11, wherein the polyimide film has YI in accordance with ASTM E313 of 15 or less; a haze in accordance with ASTM D1003 of 2 or less; and a total light transmittance in a region of 380 to 780 nm in accordance with ASTM D1746 of 80% or more.
 15. The polyimide film of claim 11, wherein the polyimide film has a modulus in accordance with ASTM D882 of 5.0 or more and an elongation of 15% or more.
 16. The polyimide film of claim 11, wherein the polyimide film has a coefficient of thermal expansion of 50 ppm/° C. or less at 100 to 450° C.; YI in accordance with ASTM E313 of 15 or less; a haze in accordance with ASTM D1003 of 2 or less; an average light transmittance in a region of 380 to 780 nm in accordance with ASTM D1746 of 80% or more; and a modulus in accordance with ASTM D882 of 8.0 or less and an elongation of 15% or more.
 17. A multilayer structure comprising the polyimide film of claim
 11. 18. A photoelectric device comprising the polyimide film of claim 11 as a flexible substrate.
 19. A flexible display comprising the polyimide film of claim 11 as a flexible substrate.
 20. A method for preparing a polyimide film comprising: applying and coating the polyimide precursor solution of claim 1 on a substrate and then performing a heat treatment.
 21. The method for preparing a polyimide film of claim 20, wherein the heat treatment includes: a first heat treatment performed at 100° C. or lower; a second heat treatment performed at higher than 100° C. and 300° C. or lower; and a third heat treatment performed at higher than 300° C. and 500° C. or lower.
 22. The method for preparing a polyimide film of claim 20, wherein the polyimide precursor solution includes 10 to 13 wt % of a solid content, based on a total weight. 